Coking is a carbon rejection process that is commonly used for upgrading of heavy oil feeds and/or feeds that are challenging to process, such as feeds with a low ratio of hydrogen to carbon. In addition to producing a variety of liquid products, typical coking processes can also generate a substantial amount of coke. Because the coke contains carbon, the coke is potentially a source of additional valuable products in a refinery setting. However, fully realizing this potential remains an ongoing challenge.
Coking processes in modern refinery settings can typically be categorized as delayed coking or fluidized bed coking. Fluidized bed coking is a petroleum refining process in which heavy petroleum feeds, typically the non-distillable residues (resids) from the fractionation of heavy oils are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically 480° C. to 590° C., (˜900° F. to 1100° F.) and in most cases from 500° C. to 550° C. (˜930° F. to 1020° F.). Heavy oils which may be processed by the fluid coking process include heavy atmospheric resids, petroleum vacuum distillation bottoms, aromatic extracts, asphalts, and bitumens from tar sands, tar pits and pitch lakes of Canada (Athabasca, Alta.), Trinidad, Southern California (La Brea (Los Angeles), McKittrick (Bakersfield, Calif.), Carpinteria (Santa Barbara County, Calif.), Lake Bermudez (Venezuela) and similar deposits such as those found in Texas, Peru, Iran, Russia and Poland.
Fluidized coking is carried out in a unit with a large reactor containing hot coke particles which are maintained in the fluidized condition at the required reaction temperature with steam injected at the bottom of the vessel with the average direction of movement of the coke particles being downwards through the bed. The heavy oil feed is heated to a pumpable temperature, typically in the range of 350° C. to 400° C. (˜660° F. to 750° F.), mixed with atomizing steam, and fed through multiple feed nozzles arranged at several successive levels in the reactor. Steam is injected into a stripping section at the bottom of the reactor and passes upwards through the coke particles descending through the dense phase of the fluid bed in the main part of the reactor above the stripping section. Part of the feed liquid coats the coke particles in the fluidized bed and is subsequently cracked into layers of solid coke and lighter products which evolve as gas or vaporized liquid. Reactor pressure is relatively low in order to favor vaporization of the hydrocarbon vapors which pass upwards from dense phase into dilute phase of the fluid bed in the coking zone and into cyclones at the top of the coking zone where most of the entrained solids are separated from the gas phase by centrifugal force in one or more cyclones and returned to the dense fluidized bed by gravity through the cyclone diplegs. The mixture of steam and hydrocarbon vapors from the reactor is subsequently discharged from the cyclone gas outlets into a scrubber section in a plenum located above the coking zone and separated from it by a partition. It is quenched in the scrubber section by contact with liquid descending over sheds, A pumparound loop circulates condensed liquid to an external cooler and back to the top shed row of the scrubber section to provide cooling for the quench and condensation of the heaviest fraction of the liquid product. This heavy fraction is typically recycled to extinction by feeding back to the coking zone in the reactor.
The coke particles formed in the coking zone pass downwards in the reactor and leave the bottom of the reactor vessel through a stripper section where they are exposed to steam in order to remove occluded hydrocarbons. The solid coke from the reactor, consisting mainly of carbon with lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel, iron, and other elements derived from the feed, passes through the stripper and out of the reactor vessel to a burner or heater where it is partly burned in a fluidized bed with air to raise its temperature from 480° C. to 700° C. (˜900° F. to 1300° F.) to supply the heat required for the endothermic coking reactions, after which a portion of the hot coke particles is recirculated to the fluidized bed reaction zone to transfer the heat to the reactor and to act as nuclei for the coke formation. The balance is withdrawn as coke product. The net coke yield is only about 65 percent of that produced by delayed coking.
The Flexicoking™ process, developed by Exxon Research and Engineering Company, is a variant of the fluid coking process that is operated in a unit including a reactor and a heater, but also including a gasifier for gasifying the coke product by reaction with an air/steam mixture to form a low heating value fuel gas. A stream of coke passes from the heater to the gasifier where all but a small fraction of the coke is gasified to a low-BTU gas (˜120 BTU/standard cubic feet) by the addition of steam and air in a fluidized bed in an oxygen-deficient environment to form fuel gas comprising carbon monoxide and hydrogen. In a conventional Flexicoking™ configuration, the fuel gas product from the gasifier, containing entrained coke particles, is returned to the heater to provide most of the heat required for thermal cracking in the reactor with the balance of the reactor heat requirement supplied by combustion in the heater. A small amount of net coke (about 1 percent of feed) is withdrawn from the heater to purge the system of metals and ash. The liquid yield and properties are comparable to those from fluid coking. The fuel gas product is withdrawn from the heater following separation in internal cyclones which return coke particles through their diplegs.
The Flexicoking process is described in patents of Exxon Research and Engineering Company, including, for example, U.S. Pat. No. 3,661,543 (Saxton), U.S. Pat. No. 3,759,676 (Lahn), U.S. Pat. No. 3,816,084 (Moser), U.S. Pat. No. 3,702,516 (Luckenbach), U.S. Pat. No. 4,269,696 (Metrailer). A variant is described in U.S. Pat. No. 4,213,848 (Saxton) in which the heat requirement of the reactor coking zone is satisfied by introducing a stream of light hydrocarbons from the product fractionator into the reactor instead of the stream of hot coke particles from the heater. Another variant is described in U.S. Pat. No. 5,472,596 (Kerby) using a stream of light paraffins injected into the hot coke return line to generate olefins. Early work proposed units with a stacked configuration but later units have migrated to a side-by-side arrangement.
Although the fuel gas from the gasifier can be used for heating, due to the low energy content, burning of the fuel gas for heat can still represent a relatively low value use for the carbon in the fuel gas. Similarly, the flue gas generated from the burner of a fluidized coking process has traditionally been viewed as a low value product. Additionally, the primary product stream derived from fluidized coking (including Flexicoking™) typically includes gases that are formed during the coking process. After separation of liquid products formed during coking, the remaining gases represent yet another product that is traditionally viewed as a low value product. What is needed are systems and methods that can allow for generation of still higher economic value products from the various gas products generated during fluidized coking type processes.
U.S. Pat. Nos. 9,090,525 and 4,899,002 describes system and methods for converting methanol to light olefins and/or conversion of methanol to gasoline boiling range compounds, and optionally distillate boiling range compounds via an olefinic intermediate.